This proposal is designed to examine the biological bases of a developmental change in latent inhibition (LI) of the classically conditioned eyelid response. Previous data from our lab suggests that conditioning emerges gradually between postnatal day 17 (PND17) and PND24 (Stanton, Freeman, & Skelton, 1992; Freeman, Spencer, Skelton, & Stanton, 1993). Recent work in infants in our lab makes it possible to look at not only the behavioral and the neurological development of eyelid conditioning in the rat which is known to be dependent on the cerebellum but to further extend this analysis to other brain structures, such as the hippocampus, and what their role might be in the modulation of higher order forms of learning, such as (LI), within this late developing learning system. In order to address these issues preliminary studies of LI have been carried out in our lab to begin to establish the ontogeny of this behavior. We have shown that while 20-day-old pups do not show latent inhibition, 24-and 32-day-old rats do. We propose to perform selective medial septal lesions and afterwards test pups on LI to see if this behavior is disrupted. The ontogeny of this behavior may reflect a developmental interaction between the hippocampus and cerebellum.